The Ly-6 alloantigens have been shown to participate in the process of T cell activation based on the ability of anti-Ly-6 mAb to induce IL-2 production and proliferation of T lymphocytes. In the present investigation we have demonstrated that peripheral T lymphocytes from A strain mice exhibited abnormally low proliferative responses after stimulation through Ly-6A/E and Ly-6C molecules when compared to responses of T cells from numerous other mouse strains. The abnormal activation of the Ly-6 pathway of A strain T cells was not due to ineffective FcR cross-linking of the anti-Ly-6 mAb, to inappropriate cellular expression of the Ly-6A/E alloantigen in A strain T cells, or to an active suppressive phenomenon. T lymphocytes from A strain mice proliferated normally when the cells were activated by mAb to Thy-1 or the CD3/TCR complex suggesting that A strain mice did not exhibit a generalized T cell activation defect. Cell separation studies of T cells and accessory cells demonstrated that this defect was quantitative, rather than qualitative, and that it was complex, residing at both the T cell and accessory cell levels. These results suggest that activation of T lymphocytes via the Ly-6 molecule may involve a signaling pathway and/or cell-cell interactions distinct from those required for optimal activation via CD3/TCR.